Is there an area that we could have a rehearsal supper? We would need an area that is covered somehow in case of bad weather.

Yes just talk with the sea people at the water's edge there are large underwater structures they will rent out if you ask nicely

They have day use gazebos that can be reserved, not sure they'd be big enough but could be a great option

They have a big yurt that could Accommodate probably 5 to 10 people but you didn't post how many people you were looking to accommodate. Otherwise you could rent A-tent and find a place within the park to put that tent up to accommodate the amount of people that you're looking for to accommodate.